caching in local operators and dofvectors

examples/CMakeLists.txt

examples/stokes3.cc

src/amdis/LocalBasisCache.hpp

src/amdis/LocalBasisEvaluatorCache.hpp

src/amdis/assembler/ConvectionDiffusionOperator.hpp

src/amdis/assembler/SecondOrderGradTestGradTrial.hpp

src/amdis/assembler/StokesOperator.hpp

src/amdis/assembler/ZeroOrderTest.hpp

src/amdis/gridfunctions/DOFVectorView.inc.hpp

src/amdis/utility/ConcurrentCache.hpp